1. Field of the Invention
The present invention relates to oscillation damping in systems having electric motors for moving and positioning a load, such as a nose wheel of an airborne vehicle.
2. Description of the Related Art
For a long time, hydraulic systems have been used to move and position various components of aircraft and other vehicles and devices. One application of hydraulics has been in aircraft nose wheel steering (moving the aircraft's front wheel to a desired angular position). Nose wheels, like many other components, can be subject to shimmy or oscillation. These oscillations, particularly when at or near the resonant frequency, can cause damage to system components, reduce their life, weaken control and hinder accuracy. Accordingly, it is desirable to dampen vibrations of the load. Damping is often achieved when hydraulic power is used by forcing hydraulic fluid through an orifice.
It is desirable to use electric motors and in particular electromechanical actuators (EMAs) to move a nose wheel or other load. EMAs have no hydraulic fluid, so another form of damping is required. Unlike hydraulic actuators, EMAs intrinsically require a power train with a large gear ratio to deliver to the load the force (or torque) required by many applications, such as a steering actuator. It has been found that such a high gear ratio causes the system to be generally unresponsive to damping. In addition, conventional wisdom is that the requirement of power typically results in use of an electric motor with a stator diameter to rotor diameter ratio of equal to or about 2 to 1 or less.
One example of how damping has been attempted involves placing elastomeric damping elements in selected positions on the nose wheel assembly, such as in U.S. Pat. No. 6,247,687.
Oscillation damping with an electric motor such as in an EMA may be needed in nose wheel steering on an aircraft, such as a commercial aircraft, in shimmy damping of any vehicle steered with EMA or an electric motor, and any other electric motor or EMA actuation system for a load where load damping is used or required.